Currently rating locks are only obeyed when using the rating: metatag.
They aren't obeyed when:
* Changing the rating via the API.
* Changing the rating via 'Rate Safe' in the mode menu (uses the API).
* Reverting to previous versions.
Also, the current behavior is to ignore the rating: metatag if the post
is locked. This patch instead makes the update fail completely (note that
this could affect trying to mass revert posts that may be rating locked).
Note: the check for `!is_rating_locked_changed?` is so that
PUT /posts/1.json?post[rating]=s&post[is_rating_locked]=true
works (ie., locking and changing the rating at the same time is okay).
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